container/std: syscall JSON adapter

This provides cross-platform JSON adapter for syscall number.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/std/seccomp.go

@@ -1,5 +1,10 @@
 package std
 
+import (
+	"encoding/json"
+	"strconv"
+)
+
 type (
 	// ScmpUint is equivalent to C.uint.
 	ScmpUint uint32
@@ -19,20 +24,53 @@ type (
 	// ScmpArgCmp is equivalent to struct scmp_arg_cmp.
 	ScmpArgCmp struct {
 		// argument number, starting at 0
-		Arg ScmpUint
+		Arg ScmpUint `json:"arg"`
 		// the comparison op, e.g. SCMP_CMP_*
-		Op ScmpCompare
+		Op ScmpCompare `json:"op"`
 
-		DatumA, DatumB ScmpDatum
+		DatumA ScmpDatum `json:"a,omitempty"`
+		DatumB ScmpDatum `json:"b,omitempty"`
 	}
 
 	// A NativeRule specifies an arch-specific action taken by seccomp under certain conditions.
 	NativeRule struct {
 		// Syscall is the arch-dependent syscall number to act against.
-		Syscall ScmpSyscall
+		Syscall ScmpSyscall `json:"syscall"`
 		// Errno is the errno value to return when the condition is satisfied.
-		Errno ScmpErrno
+		Errno ScmpErrno `json:"errno"`
 		// Arg is the optional struct scmp_arg_cmp passed to libseccomp.
-		Arg *ScmpArgCmp
+		Arg *ScmpArgCmp `json:"arg,omitempty"`
 	}
 )
+
+// MarshalJSON resolves the name of [ScmpSyscall] and encodes it as a [json] string.
+// If such a name does not exist, the syscall number is encoded instead.
+func (num *ScmpSyscall) MarshalJSON() ([]byte, error) {
+	n := int(*num)
+	for name, cur := range Syscalls() {
+		if cur == n {
+			return json.Marshal(name)
+		}
+	}
+	return json.Marshal(n)
+}
+
+// SyscallNameError is returned when trying to unmarshal an invalid syscall name into [ScmpSyscall].
+type SyscallNameError string
+
+func (e SyscallNameError) Error() string { return "invalid syscall name " + strconv.Quote(string(e)) }
+
+// UnmarshalJSON looks up the syscall number corresponding to name encoded in data
+// by calling [SyscallResolveName].
+func (num *ScmpSyscall) UnmarshalJSON(data []byte) error {
+	var name string
+	if err := json.Unmarshal(data, &name); err != nil {
+		return err
+	}
+	if n, ok := SyscallResolveName(name); !ok {
+		return SyscallNameError(name)
+	} else {
+		*num = ScmpSyscall(n)
+		return nil
+	}
+}

container/std/seccomp_test.go

@@ -0,0 +1,63 @@
+package std_test
+
+import (
+	"encoding/json"
+	"errors"
+	"math"
+	"reflect"
+	"syscall"
+	"testing"
+
+	"hakurei.app/container/std"
+)
+
+func TestScmpSyscall(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		data string
+		want std.ScmpSyscall
+		err  error
+	}{
+		{"select", `"select"`, syscall.SYS_SELECT, nil},
+		{"clone3", `"clone3"`, std.SYS_CLONE3, nil},
+
+		{"oob", `-2147483647`, -math.MaxInt32,
+			&json.UnmarshalTypeError{Value: "number", Type: reflect.TypeFor[string](), Offset: 11}},
+		{"name", `"nonexistent_syscall"`, -math.MaxInt32,
+			std.SyscallNameError("nonexistent_syscall")},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("decode", func(t *testing.T) {
+				var got std.ScmpSyscall
+				if err := json.Unmarshal([]byte(tc.data), &got); !reflect.DeepEqual(err, tc.err) {
+					t.Fatalf("Unmarshal: error = %#v, want %#v", err, tc.err)
+				} else if err == nil && got != tc.want {
+					t.Errorf("Unmarshal: %v, want %v", got, tc.want)
+				}
+			})
+			if errors.As(tc.err, new(std.SyscallNameError)) {
+				return
+			}
+
+			t.Run("encode", func(t *testing.T) {
+				if got, err := json.Marshal(&tc.want); err != nil {
+					t.Fatalf("Marshal: error = %v", err)
+				} else if string(got) != tc.data {
+					t.Errorf("Marshal: %s, want %s", string(got), tc.data)
+				}
+			})
+		})
+	}
+
+	t.Run("error", func(t *testing.T) {
+		const want = `invalid syscall name "\x00"`
+		if got := std.SyscallNameError("\x00").Error(); got != want {
+			t.Fatalf("Error: %q, want %q", got, want)
+		}
+	})
+}